To answer your questions:
-
It has actually been 3 years since Adastra took place, if I am correct. The reason it’s been just 3 years from Adastra to Khemia is due to the fact that Amicus is trying to create an alliance between Adastra and Khemia, to bring peace, which is happening as this Visual Novel progresses. As stated in Adastra by Amicus, since their relationship with the Khemians have been ultimately eradicated by the war that happened between Khemia and Adastra. (If I am not mistaken, correct me if I am.)
-
I do not believe we will ever get a Marco sprite or part in a CG (likely will be the faceless/half-bodyless version we’ve seen a few times in Adastra itself if introduced in a CG), because he is mostly a faceless character. Marco isn’t necessarily a character in Adastra; the character is supposed to be a self-insert of the player.
It wouldn’t make sense to have him appear as ‘Marco’ if he is the personification of the player. Most MCs don’t receive any official sprites, only unofficial art created by the artist, fanbase, or creator.
But again, that is up to the developers of Khemia if they decide to introduce Marco into this story with his own physical form.